Recently, researchers have focused on the refinement of sputter target microstructures based on optimizing cold-working and recrystallizing heat treatments (annealing). Unfortunately, these techniques have experienced limited success for refining high-purity copper and copper alloy microstructures. Despite the relative ease of refining copper and copper alloys' microstructure, manufacturers are reliant upon conventional working and annealing to produce acceptable results. Most commercial manufacturers of copper sputter targets tend to produce sputter targets having a grain size between about 25 and 100 μm.
Koenigsmann et al. in US Pat. Pub. No. 2001/0023726 disclose the advantage of limiting a sputter target's grain size and controlling crystallographic texture orientation ratios to improve sputter uniformity. Koenigsmann's process relies upon a combination of warm working, cold working and annealing to produce high-purity copper sputter targets having a relatively fine grain size and a balanced grain structure orientation. This process successfully produces sputter targets having a grain size on the order of 20 μm.
Pavate et al., in U.S. Pat. No. 6,139,701, disclose controlling multiple target properties using conventional production processes to lower sputter target micro-arcing for high-purity copper targets. This patent suggests reducing dielectric inclusions, grain size and surface roughness to limit defects arising from field enhanced emissions.
Despite the recent improvements achieved by Koenigsmann et al. and proposed by Pavate et al., conventional copper working processes have limited success with respect to refining microstructures to ultrafine grain sizes. This is because at normal working temperatures (ambient) copper and copper alloys reach a limiting-steady-state dislocation density and subgrain or cell size. And upon annealing this structure recrystallizes into a relatively coarse grain structure.
Target manufacturers have relied upon equal channel angular extrusion (ECAE) to produce fine grain microstructures. Nakashima et al., “Influence of Channel Angle on the Development of Ultrafine Grains in Equal-Channel Angular Pressing,” Acta. Mater., Vol. 46, (1998), pp. 1589-1599 and R. Z. Valiev et al., “Structure and Mechanical Behavior of Ultrafine-Grained Metals and Alloys Subjected to Intense Plastic Deformation,” Phys. Metal. Metallog., Vol. 85, (1998), pp. 367-377 provide examples of using ECAE to reduce grain size. ECAE introduces an enormous strain into a metal without imparting significant changes in workpiece shape. In fact sputter target manufacturers have claimed an ability to use ECAE to reduce the grain size of high-purity copper sputter targets to less than 5 μm. Although this process is effective for reducing grain size, it does not appear to align grains in a manner that facilitates uniform sputtering or provide an acceptable yield—the low yield originates from the ECAE process operating only with rectangular shaped plate and thus, requiring an inefficient step of cutting circular targets from the rectangular plate.
Lo, et al., in U.S. Pat. No. 5,766,380, entitled “Method for Fabricating Randomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains and Fine Precipitates” disclose a cryogenic method for fabricating aluminum alloy sputter targets. This method uses cryogenic processing with a final annealing step to recrystallize the grains and control grain structure. Similarly, Y. Liu, in U.S. Pat. No. 5,993,621 uses cryogenic working and annealing to manipulate and enhance crystallographic texture of titanium sputter targets.